More Issues in the Wavelength-Dispersive Spectrometric Characterization of YBa2Cu3O7-δ

Development of the techniques for microscale analysis of oxygen in YBa₂Cu₃O_7-δ (Y123) is important for determining the quality of bulk and thin film Y123 used in devices. The only characteristic X-ray for oxygen (Kα) is generated by an atomic 2p→1s transition and the “2p” electrons in oxygen are actually involved in bonding and have a range in energies. The oxygen Kα peak therefore changes shape and position according to the density of electron states. Careful consideration of this for a number of oxides shows that few of these materials have O Kα peak heights (or positions) that are compatible with each other. Conventional electron microprobe analysis that employs peak centroid height as a proxy for the peak area must, therefore, produce systematic errors for many sample-standard pairings. Some of these are documented. O Kα peak areas were also documented by step counting over the entire peak and integrating. Although a greater number of consistent sample-standard pairings were obtained by this method, the overall consistency of peak area and oxygen contents is still weak. Furthermore, Y123 has a peak that is not consistent with those of any oxide standard (including other yttrium barium copper oxides). Decomposition of the scanned peaks into a series of pseudo-Voigt peaks did not produce satisfactory results as scans could only be fitted using large numbers of small peaks. This suggests that another phenomenon is influencing the peak shape. The most likely is oxygen self-absorption which is expected to be very different for different oxides with different densities of states. The oxygen self-absorption spectrum was determined for optimally doped Y123 and has a distinct shape insofar as there are no actual peaks in the spectrum but a gentle slope over about 10 eV. High-quality analyses of O in Y123 require further development of the database for O soft X-ray emission a- d absorption.